Page:Popular Science Monthly Volume 62.djvu/474

468 angled silt, firm and lumpy when dry, but a soft mud when wet. It contains some, but not much, carbonate of lime disseminated through it, and is of a yellowish brownish color. At the edge of the roof, near the outer extremity of the tunnel, the writer dug from the wall a complete cast of a river clam. The original shell had entirely disappeared,

leaving, however, the external markings sharp and clear. No other similar specimen has been discovered in the later excavations. Land shells, of three or four species, are abundant everywhere in the material. Fragments of bones of other animals than man have been discovered in the excavation, but they are indecisive in character,—a part of a bison vertebra, a mere fragment of a small artiodactyl metapodial, etc.

Additional excavations at the end of the tunnel, and a cross-section, undertaken at the instigation of Mr. Holmes, have disclosed the projecting point of carboniferous shales underlying the superincumbent drift material, between the end of the tunnel and the near-by bank of the river. Above the skeleton, the thickness of this silty material is